1. Field of the Invention
This invention relates to variable valve timing for an internal combustion engine.
2. Discussion of Prior Art
In an internal combustion engine of the four-stroke (Otto cycle) type the phase of the engine cycle during which the inlet and exhaust valves are open is usually referred to as valve timing and is quoted as the opening and closing angles for both the inlet and exhaust valves before or after the point at which the piston reaches "top dead centre" (TDC) or "bottom dead centre" (BDC). In a conventional engine these angles are fixed and do not vary over the entire engine speed and load range.
The fixed valve timing is a compromise setting over most of the engine operating range because the dynamic behaviour of the gas-flows in the cylinder and through the valves varies considerably over the entire range. For this reason the fixed valve timing can only be correct for some required engine performance characteristic (e.g. minimum exhaust emissions, maximum power, lowest fuel consumption) at one particular engine speed and load situation and over the rest of the range poorer performance must be accepted.
This behaviour of the fixed-valve-timing internal combustion engine has been known almost since its invention but the simplicity of fixed valve timing has led to its retention with individual engine designs being a compromise aimed at some particular performance characteristic in which some desirable characteristics are sacrificed for others; for example an engine design might aim for high output power at high speed at the expense of low speed tractability.
There is a great deal of published information and patents on means of varying the valve timing of an engine. Some of these are concerned with altering the phase of the camshaft with respect to the crankshaft but these suffer the disadvantage that advancing or retarding the opening of a valve has the same effect on the closing of that valve. For example advancing the opening of an inlet valve to increase the overlap with the exhaust valve for better cylinder scavenging at higher engine speeds results in that inlet valve closing earlier so limiting the fuel charge available.